Project Summary/Abstract Advanced age is the leading risk factor for Alzheimer's disease (AD). Our program is based on a central tenet of geroscience that select pathways and mechanisms are shared between advanced age and chronic disease, and knowledge of one can inform the other. The mTORC1-S6K1 kinase axis is an obesity-activated pathway that restricts longevity, and targeting this pathway extends lifespan in animal models. This pathway has been implicated in AD pathogenesis, and pre-clinical intervention studies are promising. However, pharmacological inhibition causes harmful side-effects, deterring therapeutic application. We have discovered a Cdk5-driven bifurcation of the mTORC1-S6K1 pathway that contributes to aging and adiposity. We identified a novel, triply- phosphorylated form of S6K1 (we term S6K1*) phosphorylated at two sites in the C-terminus, as well as at Thr389, the classical mTORC1 activation site. Multi-site phosphorylated S6K1 directs phosphorylation of novel targets, including the dual function tRNA synthetase, Glu-Pro tRNA synthetase (EPRS), coenzyme A synthase (CoASY), and lipocalin-2 (Lcn2). Importantly, mice bearing a phospho-deficient mutation of EPRS at the critical Ser999 residue are lean and exhibit ~120-day lifespan extension. Ser999 EPRS phosphorylation is required for elevated adipocyte expression of key longevity-related adipokines, including monocyte chemoattractant protein- 1 (MCP1) and plasminogen-activator-1 (PAI-1). MCP1 is a pro-inflammatory protein predominant in the senescence-associated secretory phenotype (SASP); PAI-1 is a marker and mediator of cell senescence and aging, and a null mutation in SERPINE1, the gene encoding PAI-1, protects against biological aging. Importantly, these novel, age-related targets of S6K1* also are implicated in AD progression. For example, serum MCP1 level is associated with cognitive decline in mouse AD models and AD patients, and PAI-1 knockout, or pharmacologic inhibition, reduces AD in mice. We hypothesize that the extended mTORC1-S6K1 pathway and its effectors contribute to AD onset and progression, and that genetic inhibition of the pathway will retard AD onset and reduce its severity with minimal adverse side effects. Also, mice fed a high-fat diet will show that obesity influences aging and AD progression by a common pathway. We will test these hypotheses in two Specific Aims. In the first Aim, we will elucidate the role of the S6K1* pathway in AD progression. AD- susceptible mice will be bred with two genetic mouse models of S6K1* pathway inhibition developed in our laboratory, namely, EPRS phospho-deficient knock-in mice bearing a Ser999-to-Ala mutation, and our newly developed S6K1 Ser429-to-Ala mouse model, that lacks the extended S6K1* substrate selection, but exhibits unaltered canonical S6K1 kinase activity. We will determine effects of S6K1* pathway inhibition on AD pathology and adverse cognitive side-effects. In the second Aim we will determine the influence of obesity on S6K1* pathway-mediated AD progression. Our program will establish new mechanisms and molecular targets for intervention in the aging process and AD.